The present invention relates to a single crystal pulling apparatus for growing and pulling up a single crystal ingot from molten liquid of a polycrystal substance based on the Czochralski (CZ) technique.
Such a single crystal pulling apparatus comprises a heating chamber wherein a single crystal is grown from polycrystal substance (a raw material such as silicon); in the heating chamber are provided such elements as a quartz crucible, a cylindrical heater surrounding the crucible, and a cylindrical thermal insulator (heat shield) surrounding the heater. Beneath the heating chamber are provided mechanisms for rotating the crucible about a vertical crucible shaft, on which the crucible is fixedly mounted, to control the uniformity of the heat flux in the molten polycrystal substance during the crystal growing operation.
The polycrystal substance charged in the quartz crucible is heated and melted down by the heater to turn into a polycrystal molten liquid (melt), and in this liquid is dipped a seed crystal fixed at the lower end of a pull means such as a wire, and the desired single crystal grows from the seed crystal as the pull means is rotated and drawn up at predetermined rates together with the seed crystal.
(Problems the Invention seeks to solve)
In such a conventional CZ technique type single crystal pulling apparatus, the resistivity gradually decreased with the progress of the growing operation due to segregation of the dopants; thus, the resistivity distribution in the ingot tended to be such that the closer to the bottom of the ingot, the lower was the resistivity. As the result, the occurrence rate of off-specification wafer was relatively high. For this reason, a continuous charge method has been adopted, according to which a measured amount of granular polycrystal substance is continuously poured over the melt in the quartz crucible while the single crystal ingot is pulled up.
However, the powdery polycrystal substance floats on the surface of the melt as the granular polycrystal substance is poured over the melt, and there is a strong tendency that the natural convection in the free surface of the melt carries the powdery polycrystal substance toward the single crystal from the crucible, and as the result, the powdery polycrystal substance enters the solid/melt (growth) interface of the single crystal thereby causing dislocations in the single crystal.
Also, owing to the temperature difference between the granular polycrystal substance and the melt, localized temperature gradients are created in the melt resulting in dislocations in the single crystal.
Therefore, it is impossible to grow a flawless single crystal using a single crucible; for this reason, the duplex crucible method has been adopted, according to which a cylindrical internal crucible is concentrically provided in the quartz crucible to form a duplex structure, and an appropriate amount of granular polycrystal substance is poured outside the internal crucible in a continuous manner. This duplex structure provides a shield such that the surface convection caused by heat cannot carry the powdery polycrystal toward the single crystal ingot to thereby prevent the dislocation in the single crystal caused by floating polycrystal powder.
However, in this conventional duplex crucible method, the internal crucible was fixed to the external crucible by welding or by some other means so that the internal crucible was immovable relative to the external crucible, and as such the internal crucible was always submerged in the melt; for this reason, the internal crucible had to be made of a special material and fixed to the external crucible with high technology so that it could undergo thermal deformation without breaking and detaching from the external crucible, and this made the cost for providing the internal crucible very high.
Thus, in place of the internal crucible, a cylindrical partition is provided and is dipped in the melt, separating the melt surface into the internal single crystal growth region and the external raw material supply region. This partition also prevents the powdery polycrystal from being carried toward the single crystal ingot to thereby prevent the dislocation in the single crystal, and furthermore the partition prevents the thermal disorder in the melt in the vicinity of the single crystal so as to further minimize the
crystal dislocation (ref. Japanese Patent Kokoku No. 59-50627).
As the material for the cylindrical partition, silicon nitride or opaque quartz glass has been used; however, silicon nitride is over expensive, and the opaque quartz glass, which is of the same material as is used to make the crucible, contains bubbles in it so that when the partition is dipped in the super-hot melt, the bubbles expand in the quartz glass whereby that part of the partition which is dipped in the melt undergoes severe corrosion and degradation, and fragments of corroded quartz glass fall on the melt surface on which it floats and reach the growth interface to interfere with the smooth single crystallization.
Also, with the structure of the conventional single crystal pulling apparatus, since there was no shield between the melt surface and the inner walls of the heating chamber, the heat of the melt was radiated directly to the inner walls of the heating chamber; thus, the melt surface, especially the interface joining the internal crucible and the melt surface, cooled and the melt tended to solidify at the interface and, as the result, it was difficult to increase the single crystal growth rate. Furthermore, the silicon monoxide (SiO) evaporating from the melt surface deposited itself on the inner walls of the chamber and eventually caused dislocations in the growing crystal.
There is a control method called melt amount reduction type control method, in which the melt amount is reduced as the single crystal is pulled up so as to control the dopant concentration in the single crystal. In the case of adopting such control method, the area by which the melt contacts the crucible is reduced so that in proportion to this the amount of the oxygen that dissolves into the melt from the crucible is reduced and as the result the oxygen concentration of the single crystal gradually decreases with the crystal growth, making the single crystal biased.
The present invention was made in view of the above problems, and it is, therefore, an object of the invention to provide a single crystal pulling apparatus which is designed such that it can produce high quality single crystal ingots having scarce dislocation at high productivity, and such that the life of the crucible is extended greatly to reduce the running cost.